1. Field of the Invention
The present invention relates to an image pickup apparatus, a control method therefor, and a storage medium storing a control program therefor. It particularly relates to a technique for generating one image by composing a plurality of images.
2. Description of the Related Art
In recent years, many digital still cameras and digital video cameras using solid state image pickup devices, such as a CCD and a CMOS, are developed.
A dynamic range of a shot image in such a camera is usually dependent on a dynamic range of a solid state image pickup device. Whiteouts or blackouts occur in a background even if an exposure of a main object is correctly adjusted under a condition where contrast between the main object and the background is excessive due to backlight.
Japanese laid-open patent publication (Kokai) No. S63-306779 (JP 563-306779A) discloses a technique that expands a dynamic range by composing a plurality of images in order to solve such a problem, for example.
The technology substantially expands the dynamic range of a shot image in a video camera with a CCD image sensor by composing image signals of different storage time captured alternately so that the whiteout portions in a high exposure image with the long storage time and the blackout portions in a low exposure image with the short storage time are compensated by the other image signals.
FIG. 7 is a timing chart showing driving timing of an image pickup device and image output timing after composing images in a conventional general dynamic range expanding method.
In FIG. 7, an output synchronizing signal indicates the timing at which the image after composing the images is outputted, and its period corresponds to a frame rate of the image output. A shooting synchronizing signal indicates the timing at which an image of one frame is shot, and its period corresponds to a frame rate of the image shooting. In this example, the period of the output synchronizing signal is twice the period of the shooting synchronizing signal.
Synchronizing with the shooting synchronizing signal, the high exposure image High and the low exposure image Low are alternately acquired, and the image with expanded dynamic range HDR that is generated by composing the high and low exposure images is outputted as the following frame. An exposure change process is executed at the same timing as that for the shooting synchronizing signal.
In the acquisition method shown in FIG. 7, the high exposure condition that is higher than the correct exposure condition by one step is calculated based on the correct exposure condition of the subject acquired from an automatic exposure (AE) process at the time T1, and a high exposure image is shot under the calculated high exposure condition.
At this time, an exposure change process is executed as shown in FIG. 7. The exposure change process includes processes required for a switching direction to the exposure time of the image pickup device, a switching direction to a signal amplification amount, a change direction to an aperture value, etc, and accompanying operations of the image pickup device and the diaphragm mechanism, etc. in addition to the calculation process of a CPU.
At the following time T2, the low exposure condition that is lower than the correct exposure condition by one step is calculated, and a low exposure image is shot under the calculated low exposure condition. At this time, the exposure change process is executed as shown in FIG. 7.
At the following time T3, the image with expanded dynamic range that is generated based on the high exposure image shot in the second frame prior to the current frame and the low exposure image shot in the immediately preceding frame is outputted as a moving image. At the same time, the exposure change process is executed and the high exposure image of the following frame is shot like at the time T1.
As mentioned above, the images are acquired in order to generate an image with expanded dynamic range by repeating the operation that changes the high exposure image shooting and the low exposure image shooting, in the speed that is twice the frame rate for the output moving image.
However, the prior art mentioned above requires the reading speed as high as possible in order to reduce a gap of the exposure times of composite images. This increases the load of an image composing process, and a control accompanying high-speed reading, especially the exposure change process depending on the high-speed reading suppresses other various processes executed by the CPU.